Shouldn't the Mario characters come from Mario Kart?
Fire Emblem is popular in Japan, but it's far from the biggest. Last year alone Mario, Splatoon, Animal Crossing, and Rhythm Heaven sold more as far as Nintendo games are concerned. Corrin is just in because Sakurai thought they were cool (to be fair, they are) and it was convenient advertising in the west. Nothing about FE being Zelda tier.